Coffee is a cornucopia of chemical compounds. Together, they create a unique composition that gives coffee its unique smell, flavourful taste and kick of energy. So what does chemistry have to do with all of this?

Chemical Composition of Coffee

Coffee contains a variety of different chemicals, including over a thousand aromatic compounds. Of course, caffeine is the most well-known participant in coffee – it’s the reason why most adults drink it in the first place, and we’ll get into how it works further down the page.

But the chemical composition of coffee is also made up of many other compounds:

  • Quinic Acid: this is obtained from many plant sources including cinchona bark and, you guessed it, coffee beans. It is also what gives coffee its acidity.
  • 3,5 Dicaffeoylquinic Acid: in laboratories, this has been used to pre-treat neurons. It has proven antioxidant effects by protecting cells from damage caused by free radicals.
  • Acetoin: this compound has a rich buttery smell that it lends to coffee. It is also used as a flavour additive in butter and microwavable popcorn.
  • Putrescine: produced by the breakdown of amino acids in living and dead organisms, this foul-smelling compound actually occurs naturally in coffee beans.
  • Trigonelline: this molecule is important because it breaks down into pyridines which give coffee that sweet, earthy taste. It also stops bacterium from decaying your teeth.
  • Chlorogenic Acids (CGA): approximately 8% of unroasted coffee beans are made up of this group of compounds, which contribute to the characteristic bitterness of coffee.

Aromatic Compounds

There are trillions of molecules in the air around a cup of coffee, and the hot vapour that rises from the beverage can carry thousands of different molecules. The aroma of coffee can be attributed to a few of these aromatic molecules:

  • Pyrazine smells earthy
  • Methylpropanol smells fruity or spicy
  • Methional smells like a baked potato
  • Methanethiol smells like cabbage or garlic

Most of these compounds are made when coffee beans are roasted, an action that provides the energy required to convert the bitter chlorogenic acids into a fragrant drink. However, no matter how much conversion takes place, that bitter characteristic will always be a part of coffee.

Hands cupped together holding coffee beans
There are a huge range of compounds that make up the chemical composition of coffee. These all contribute to the characteristic smell and flavour of this beverage.

Why Does Coffee Taste Bitter?

Many people think that the bitter taste of coffee is caused by caffeine. While this is a dominant chemical in coffee, only 15% of bitterness is caused by caffeine. The bitter taste of coffee is a result of two pungent and antioxidant compounds found in roasted coffee beans: chlorogenic acid lactones and phenylindanes.

Chlorogenic Acid Lactones

Chlorogenic acids (CGA) are esters of caffeic acid and quinic acid. The term ‘chlorogenic acids’ actually refers to a polyphenol family of esters, including quinic acid and:

  • Hydroxycinnamic Acids: this refers to a class of aromatic acids like caffeic acid, ferulic acid and p-coumaric acid. They are all hydroxyl derivatives of cinnamic acid, which has an odour that is similar to honey.

Despite the name, chlorogenic acids do not contain chlorine. The name actually comes from the Ancient Greek word chlorós, meaning light green. This is because of the colour that is produced when these group of acids are oxidised.

When CGAs dehydrate, they form bitter-tasting molecules called chlorogenic acid lactones, which is ironic considering that CGA itself is not bitter.

Two dominant chlorogenic acid lactones appear in high concentrations in light to medium coffee brews: 3-caffeoylquinic-1,5-lactone and 4-caffeoylquinic-1,5-lactone. As lactones, these compounds are the dominant source of a bitter taste in coffee.


Phenylindanes are the chemical breakdown products of chlorogenic acid lactones, and they are found in much higher concentrations in dark coffee brews – like espressos.

Phenylindanes leave a much harsher bitter taste than their precursors, and this explains why dark-roasted coffee brews are much more intense beverages.

A birds-eye shot of different strengths of coffees around a circular wooden tray
Coffee tastes bitter because of the presence of chlorogenic acid lactones and phenylindanes. These are the chemical compounds that release the bitter taste in a cup of coffee.

The Chemistry of Caffeine

One of the reasons why adults all over the world love their coffee is because of caffeine, nature’s miracle drug that gets you ready for that 9 a.m. meeting on Monday morning. In a nutshell, caffeine works by tricking your adenosine receptors in your brain.

What is Adenosine?

Adenosine has many physiological roles. In the brain, it is an inhibitory neurotransmitter. This means that it behaves like a central nervous system depressant by doing things like promoting sleep and suppressing arousal.

Adenosine functions by binding to adenosine receptors in the brain. However, when you drink coffee, caffeine can also bind to these receptors. To a nerve cell, caffeine looks a lot like adenosine, which is why it is able to bind to so readily.

Caffeine in Disguise

When attached to the adenosine receptors, caffeine takes up all the room so that adenosine can no longer be identified by the receptors. Therefore, instead of slowing down the activity of the nerve cells like adenosine does, caffeine speeds everything up.

This also increases neuron firing in your brain. The pituitary recognises this activity as an emergency and releases hormones to tell the adrenal gland to produce adrenaline. All of this is why, when you drink a lot of coffee, you generally feel more awake and sometimes shaky: the presence of caffeine is essentially rewiring the function of your brain by suppressing adenosine.

A similar process happens when you eat chocolate, which contains theobromine. This compound is also able to block adenosine receptors, and this is one of the reasons why chocolate can be addictive.

Two cups of coffee on a dark wooden table surrounded by succulents
Caffeine works by tricking the adenosine receptors in the brain. When it does this, it speeds up cell activity and causes hormones to release adrenaline. This is why coffee and even chocolate can be so addictive.

The Perfect Brew

There are several variables that can be controlled to create the perfect brew. Of course, the type of brewing device influences the standard of coffee. Less obviously is the effect that water has on the final brew.

Water Chemistry

The presence of compounds like quinic acid and chlorogenic acids make coffee a fairly acidic beverage. Therefore, when making the perfect brew, it is important to take into account the acidity of water being used.

It has to have the perfect acidic balance in order to create a brew that tastes like it has come straight out of Starbucks. Water chemistry is also why, no matter how hard you try, making an at-home brew never really tastes as good as the one made by your local barista. There are two types of water available from your tap:

  1. Soft water contains low levels of calcium ions and, most importantly, bicarbonate. This results in a very acidic cup of coffee.
  2. Hard water contains high levels of bicarbonate, which neutralises the flavoursome acids in coffee and results in a chalky taste.

Finding the right water chemistry that is somewhere in the middle of these two extremes is the key to making the perfect brew.

A sack of coffee beans spilling onto a coffee cup
Creating the perfect coffee brew all comes down to several factors like particle size distribution and the type of brewing device. But water chemistry also plays a huge role.

Milk and Coffee

One of the most satisfying parts of making a cup of coffee is pouring the milk and watching as it swirls in rapid cloud-like formations throughout the coffee.

The blooming effect that happens when milk first enters the coffee is first caused by the force of gravity, which pulls the milk to the bottom of the mug. This motion is then disrupted by the stickiness of the coffee and milk molecules interacting with each other. The walls of the mug act as a surface for this swirling motion to bounce off, creating those beautiful upward spirals.

Milk finally mixes with coffee because of diffusion. Diffusion is when particles move from an area of high concentration to an area of low concentration until they are eventually spread evenly. It is the same process that allows breathing to keep us alive, and it is how milk diffuses itself throughout the coffee and mixes into it.

Milk being poured into coffee in a glass kilner jar
The blooming swirl effect that happens when milk is poured into coffee is caused by the force of gravity as well as the stickiness of milk and coffee molecules.

There’s a lot more chemistry going on inside your average cup of coffee than you may realise. ReAgent may not be able to supply you with top-level coffee, but we do stock hundreds of high-quality chemical products to suit your business needs. Check out our online store where you can place your order in seconds.  


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